In the episode when the Simpsons go to Africa, they are on a boat. Homer sticks his hand in the water and gets a bunch of leeches on him. The camera turns, and when it comes back, the leeches are gone!
Yeah, I noticed this to. I was going to post it as well, but I wasn't able to, there were too many slip-ups waiting to be seen. But who cares? This is a great slip-up.